Induction motor control system

ABSTRACT

This disclosure relates to a control system for dynamically reenergizing a rotating induction motor. A pair of inverters supply power to the windings of the induction motor through an inductive reactor. A tachometer senses the rotational speed of the rotor of the induction motor and applies a signal to a programmed logic circuit for pulse width modulating the power of the inverters over a constant torque range and for step wave shaping the power of the inverters over a constant horsepower range. In the event of an interruption of power, the reapplied power to the induction motor is reduced and the frequency of the reapplied power is readjusted at or near the actual synchronous speed of the induction motor so that little, if any, current surges occur upon reenergizing of the induction motor.

I United States Patent 1191 1111 3,781,616 Mokrytzki et a1.

[ Dec. 25, 1973 INDUCTION MOTOR CONTROL SYSTEM Primary Examiner-Gene Z.Rubinson [75] Inventors, Boris Mokrytzlu, Highland Heights,

Peter w. Hammond, Chagrin, both Attorney-R. W. Mclntire, Jr. et al.

of Ohio [73] Assigneez Westinghouse Air Brake Company, I 57] ABSTRACTWilmerdmg, Pa. [22] Filed: Oct. 13, 1971 This disclosure relates to acontrol system for dynami- I I cally reenergizing a rotating inductionmotor. A pair [2!] Appl' 188949 of inverters supply power to thewindings of the induction motor through an inductive reactor. A tachome-[52] US. Cl 318/230, 318/231, 318/318, ter senses the rotational speedof the rotor of the in- 318/340, 318/388, 318/392, 318/404, 318/415duction motor and applies a signal to a programmed [51] Int. Cl. [102p1/30 logic circuit for pulse width modulating the power of [58] Field ofSearch 318/230, 231, 308,

the inverters over a constant torque range and for step 318/318, 336,340, 388, 392, 393, 404, 415, wave shaping the power of the invertersover a con- 416 stant horsepower range. In the event of a interruptionof power, the reapplied power to the induction motor [56] ReferencesCited is reduced and the frequency of the reapplied power is UNITEDSTATES PATENTS readjusted at or near the actual synchronous speed of 3477 002 II 969 CampbeII 3I8/23OX the induction motor so that little, ifany, current 2 663 834 12 1953 Large et 11 12:: I: 318/416 x Surgesenergizing the inductim 3,584,276 6/1971 Ringland et al 318/231 x IFOREIGN PATENTS OR APPLICATIONS 22 CI 5 Drawing Figures 1,160,939 8/1969Great Britain 318/231 1 I LA 26 27 i 3 I 11 1 1 1 1 8 1 1 1 1 PULSE 1 II W I WIDTH l 46 I THREE LB 31 32 I l r W 1 1 1 1 i '64 EQfiIg I I I IW3 W2 I 1 I 1 1c 36 37 I 1 I /gogglsoov l I af 1 1 I43 LA 28 29 2 I 42 1T I I i 31 PSJEEP 3% I I +15 I I I 4 I FORMING 47 I THREE LB 33 34 23AND gHAsE 1 I PHASE I I I AN LE 1 RTER CONTROL 53 I I M W6 W5 I 1 o ;1c1 I I I I I NETWORK I I 38 39 24I 5 1 1 i l 1 F 54 I I4 I I3 I g I 11 II Low-PAss I I w I I I 1 FILTER I I I I e 1 1Q 1 MODE 58| I I SELECTOR 1I I 571 HIGH-PASS I I so I FILTER I I I69 68 L l RING I I2 7 1 VTFC 67d1 PATENTED BEL 2 51575 SHEET 10F 5 w EUE -OIUS.

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ATTORNEY PATENTED 3.781.616

SHEET l BF 5 VAB BORIS mok iyz r i 7 1 4 BY PETERW HAMMOND g w- ATTORNEYINDUCTION MOTOR CONTROL SYSTEM This invention relates to a motor controlsystem and, more particularly, to a system for reenergizing a rotatinginduction motor in motion by sensing the speed of the motor and byreapplying a lower than normal level of a.c. power having a frequencywhich is equal to the frequency representative of the synchronous speedof the motor at the time the power is reapplied to the motor.

In railroad and mass and/or rapid transit operations, it is commonpractice to employ electrical power as the source of energy. In certaininstallations, power in the form of dc. voltage is carried by overheadlines or by a third rail which parallels the track rails. Normally,electrical pantographs or conductive shoes pick up the dc. power fromoverhead or wayside conductors and apply it to the vehicle-carriedequipment. Naturally, the power is used for illumunation,air-conditioning, heating controls and, of course, for the propulsion ofthe vehicles. The vehicles are propelled by electric motors which drivethe wheels and move the vehicles along their routes of travel. It hasbeen found advantageous to employ lightweight, high torque, highlyefficient, and less costly a.c. induction motors to drive the vehicles.However, in do propulsion territory, it is necessary to convert the dc.voltage into an ac. voltage for use by the motors. The conversion isaccomplished by static inverters which transform the dc. voltage into anac. voltage. In a three-phase dual-winding induction motor propulsionsystem, it is highly advantageous to utilize two bands of three-phaseinverters. With a conventional pulse width modulation and six-step waveshape propulsion system it is possible to obtain torquespeedcharacteristics which are equal to that of a dc. traction motor systembut without the need of brushes and mechanical commutators. However, aninduction motor propulsion system is capable of developing hightransients or surges under certain conditions which could impair theoperation of the system. For example, if the vehicle is in a runningmode of operation and if another vehicle in the vicinity is suddenlybrought to a stop, high voltage surges will be induced into the powersupply of the running vehicle.

In order to prevent damage to the inverters, it is necessary to tripoutor remove the power to the motor control system. The inverters are alsoincapable of withstanding high current surges without chance of damageor destruction and, therefore, a tripout or removal of power takesplace. Other faults occur due to human failings which result in the lossof power or excitation to the induction motor. Now, if the stator of thepropulsion motor retains any appreciable amount of residual magnetism atthe time of the loss of power, the reapplication of full load powercould result in very high current demands. That is, if the magneticpoles are at or near saturation and if the instantaneous value of thereapplied supply voltage is at or near its peak value, the load currentcould exceed eight to times its normal level. When the frequency of theapplied voltage does not match the frequency of the rotating propulsionmotor when the system is reset after a loss of power, currentsproportional to torques will also be drawn. It will be appreciated thatthe rotational speed of an induction motor is proportional to thefrequency of the ac. supply voltage. Thus, a supply voltage frequencywhich is higher or lower than the frequency of the rotating motor willunduly load the motor so that an additional amount of current will bedrawn during an attempted restart. These high current surges are capableof interrupting the operation of the system in that the protectivedevices, such as, the fuses or circuit breakers are opened with theinevitable result of system shutdown. Previously, the moving vehicle hadto be brought to a complete stop before restarting when a loss of motorexcitation occurred in order to ensure that the propulsion system wouldnot be disabled. It will be appreciated that if the vehicle is requiredto be stopped each time a loss of excitation through fault occurs, theresulting service and efficiency of the transportation system is greatlyreduced. Thus, the advantages of an induction motor propulsion systemcan not be effectively realized unless the problems of the occurrence ofloss of excitation faults can be resolved.

Accordingly, it is an object of this invention to provide an inductionmotor control system which need not be brought to zero speed forresetting.

Another object of this invention is to provide an induction motorpropulsion system which is capable of being restarted in motion withoutincurring the possibility of system shutdown.

A further object of this invention is to provide an ac. motor controlcircuit which prevents the generation of high current surges when therotating motor is restarted by reducing the level of the voltage andadjusting the frequency of the reapplied voltage to the frequency of therotating motor in order to prevent the appearance of high surgecurrents.

An additional object of this invention is to provide a control systemwhereby a rotating induction motor may be safely restarted in motion byreducing the normal level of supply voltage and by making the frequencyof the supply voltage equal to the frequency of the rotating motor whenthe supply voltage is reapplied to the rotating motor.

Still another object of this invention is to provide a motor controlsystem employing a variable frequency and voltage inverter circuit forsupplying an ac. induction motor.

Still a further object of this invention is to provide an inductionmotor control circuit utilizing a three-phase static inverter having avariable frequency and voltage characteristics.

Still an additional object of this invention is to provide a controlsystem for dynamically restarting a rotating induction motor.

Yet another object of this invention is to provide a control circuithaving two variable three-phase inverters for controlling the amplitudeof a voltage and the frequency of the voltage supplied to the windingsof a squirrel-cage induction motor.

Yet a further object of this invention is to provide a control circuithaving a dc. supply source, a pair of three-phase inverters, a speedresponsive device and a logic circuit for dynamically restarting aninduction motor by reapplying a lower than normal level of ac. powerhaving a frequency which is equal to the frequency representative of thespeed of the rotor of the induction motor at the time that the power isreapplied to the induction motor.

Yet an additional object of this invention is to provide a motor resetcontrol circuit reliable in operation, durable in use and efficient inservice.

In accordance with the invention, the control system utilizes polyphaseinverters, an inductive reactor, a

I speed sensing device, and a programmed logic circuit for dynamicallyresetting or reenergizing a rotating induction motor. The inductionmotor includes a pair of three-phase wye connected stator windings and asquirrel-cage rotor. The inductive reactor includes a plurality of mainwindings and a plurality of auxiliary windings which are connected tothe various phases of the polyphase inverters and to the stator windingsof the induction motor. The programmed logic controls the voltage aswell as the frequency of the polyphase inverters in accordance with thespeed'of the induction motor rotor which is measured by speed sensingmeans and is applied to the programmed logic circuit. Upon normalstartup, the induction motor operates as a constant torque device sothat the voltage and frequency are varied by a pulse width modulationcontrol network of the programmed logic in accordance with the speed ofthe motor until a base speed is reached. Thereafter,

' the induction motor operates as a constant horsepower device so thatthe voltage is held constant and only the frequency is varied by asix-step control network of the programmed logic in accordance with thespeed of the motor. If a power loss occurs when the induction motor isrunning in the constant torque range, the voltage level is immediatelyreduced by the pulse width modulation control network and the speedsensing device causes the programmed logic circuits to ensure that thefrequency of the voltage of the polyphase inverters is the same as thefrequency of the induction motor at the time that power is reapplied tothe motor. If a power loss occurs when the motor'is running in theconstant horespower range, the voltage level is reduced by a phase anglecontrol network in the logic and the frequency of the induction motorand the speed sensing device causes the programmed logic circuit toensure the frequency of the voltage of the polyphase inverters is thesame as the frequency of rotation of the induction motor at the timethat power is reapplied to the motor.

Other objects, features and advantages of the present invention willbecome more apparent from the description of the preferred embodimentdescribed with reference to the accompanying drawings forming a part ofthis specification, and in which: 7

FIG. 1 is a schematic circuit diagram of a preferred embodiment of theinvention.

FIG. 2 illustrates the voltage versus the frequency characteristics foran induction motor in the fully excited and in the dynamic reset mode.

FIG. 3 is a timing diagram illustrating a resultant voltage wave formwhich is produced by notched square waves which are added in a givenphase relationship.

FIG. 4 is a timing diagram illustrating a resultant twelve-step voltagewave form which is applied to the stator windings of a dual wound a.c.induction motor.

FIG. 5 is a timing diagram illustrating a resultant voltage wave formwhich is produced by square waves which are shifted in a leading andlagging relationship with each other.

motors characterized by the numeral 11. The induction motor 1 1 includesa four-pole stator. A plurality of windings W1, W2, W3 and W4, W5, W6are wye connected to form a two or dual phase arrangement. The motor 11also includes a conventional cast aluminum squirrel-cage rotor R. Itwill be noted that the two sets of windings W1,W2,W3 and W4,W5,W6are-mechanically displaced by 30 as illustrated. The reason for thismechanical displacement is to match an inverter phase displacement tothe motor and to remove or eliminate the effect of the fifth andseventh-harmonics by a reactor section, as will be describedhereinafter.The presence of these harmonics adversely affects the operation of themotor as they create excess heating of the motor. It should berecognized that the use of three-phase operation inherently removes theeffect of the third harmonic which could also reduce motor operatingefficiency.

It should also be recognized that the 30 mechanical displacement isimportant when considering motor operation from a power factorstandpoint. This physical displacement which matches that of theinverters provides a balanced power factor in each of the inverterswhich enhances system operation in a manner that will be more readilyappreciated from the description that follows hereinafter.

An inductive reactor 12 couples the output of a system of converters,such as, a pair of three-phase inverters 13 to the respective windingsof the induction motor 11. A suitable d.c. power supply 14, such as 600volt line voltage is taken from the wayside. As shown, the inductivereactor 12 includes a plurality of main and auxiliary windings. It willbe appreciated that the main and auxiliary windings 26 and 27 as well asthe main and auxiliary windings 28 and 29 are wound upon a common leg ofthe reactors magnetic core -(not shown). In a similar fashion, the otherwindings 31 and 32, 33 and 34 are wound on a separate leg of thereactors core which also is not shown. In a like fashion, re-

maining windings 36 and 37, 38 and 39 are also wound upon a separate legof the reactors magnetic core. Auxiliary windings 32, 37, 27.arerespectively connected to motor windings W1, W2, W3 by leads 18, 19,21. In addition, auxiliary windings 39,29, 34 are respectively connectedto motor windings W4, W5, W6 by leads 22, 23, 24.

The functional operation and details of this reactor 12 are set forth inthe copending application for Letters Patent of the United States, Ser.No. 187,974, filed Oct. 12, 1971, by Udo l-I. Meier, for InverterParalleled With Reactor. The operation of this reactor 12 and itscooperation with the induction motor 11 and the pair of inverters 13will be more fully explained hereinafter.

The pair of three-phase inverters 13 are individually referenced bynumerals 41 and 42. These inverters 41, 42 are of the type shown anddescribed in Letters Patent Of the United States No. 3,207,974, issuedSept. 12, 1965 to W. McMurray, for Inverter Circuits. As mentionedabove, the inverter circuits 41, 42 are connected to terminal 43 of asuitable d.c. supply source (not shown) by double-pole single-throwswitch 44 which in actual practice may be a pushbutton type switchactuated by a motorman when it is desired to place the vehicle inservice. Interposed between terminal 43 and switch 44 is a fuse 50, thefunction of which will be explained hereinafter.

It will be noted from the written designations on inverter circuits 41and 42 that a 115 phase displacement exists between the center of theirrespective three-phase outputs. Leads LA, LB, LC connected to mainreactor windings 26, 31 and 36 constitute the output ofinverter circuit41, while leads LA, LB, LC connected to main reactor windings 28, 33 and38 constitute the output of inverter circuit 42. As previouslymentioned, keeping inverter power factors equal requires that the firingtime of inverter 41 by slightly delayed relative to the firing time ofinverter 42. That is, in order to ensure that each of the two wyeconnected stator windings of induction motor 11 has equal power factorswhich is necessary for efficient operation, the 30 phase displacement isemployed to offset the design characteristic created by the 30mechanical displacement. Accordingly, the inverter 42 is designed tohave a lead on its output with respect to the motor reference angle andinverter 41 will have a 15 lag from the motor reference angle.

A careful study of the McMurray patent will readily reveal that patenteeemploys solid state devices in his inverter circuits. Thus inverters 41and 42 which are incorporated in the presently described motor controlsystem also preferably employ solid state circuitry. It will be notedthat the inverter circuits normally utilize silicon controlledrectifiers or thyristors which are well known switching devices andtherefore are not shown in this system illustration.

The thyristors or silicon controlled rectifiers are alternately renderedconductive in a selective fashion by gating pulses received from aprogrammed logic circuit 15 via multi-wave cable leads 46 and 47.

The programmed logic circuit 15 includes a pulse width modulationnetwork 45 which feeds lead 46 via lead 48 and which also feeds lead 47via lead 49. Leads 46 and 47 are symbolic only in that they representcombined six-step signals and modulated signals which are connectedrespectively toinput gating circuits of the inverters 41 and 42. As willbe described hereinafter, the pulse width modulation network 45 controlsthe inverters during a constant torque operation of the induction motor11.

The programmed logic circuit 15 also includes a sixstep wave forming andphase angle control network 51 which feeds inverter control leads 46 and47 via leads 52 and 53, respectively. The six-step phase angle controllogic network 51, to be described more fully hereinafter, is employedduring a constant horsepower operation of the induction motor 11.

It will be seen that the programmed logic circuit 15 also includes amode selector 54 connected respectively by leads 56, 57 to pulse widthmodulation network 45 and the six-step wave forming and phase anglecontrol network 51. The mode selector includes a low pass filter 58which is connected to lead 61, via lead 62 and a high pass filter 59which is also connected to lead 61 via lead 63.

Alternatively, the inverter output mode of operation is determined bythe absolute value of motor speed; thus if speed is low when a faultoccurs requiring dynamic reset, the wave form of the inverter is thesame that it would have under normal operation except that pulse widthis drastically reduced. If the motor is in the high speed constant HPmode of operation and a fault occurs, voltage from the invertersindividually is in the six-step mode and to control voltage we mustphase shift each of the inverters away from the normal :15" position tosomething like i65 perhaps to reduce votage drastically whilemaintaining frequency and the reference phase point specifically.

Lead 61 is connected to the speed sensing device 16 which, in thepresent instance, may take the form of a conventional commerciallyavailable tachometer 64. The tachometer 64 is connected to the shaft 66of the rotor R of the induction motor 1 1. Thus, the frequency of thesignals on output lead 67 of the tachometer 64 is a function of therotational speed of the squirrel-cage rotor R. It will be appreciatedthat the speed of rotation of the rotor R is a function of the frequencyof the motor supply voltage minus the frequency of the slip normallypresent in this type of motor. Thus, the tachometer 64 produces a signalfrequency translatable to a reference which is indicative of the actualrotating frequency or speed of the rotor R. It should be noted that thelead 67 is also connected to a voltage-tofrequency converter 68 via lead67a. The voltage-tofrequency converter 68 normally operates at a muchhigher frequency than the inverter fundamental frequency and isconnected to a ring and scaling counter circuit 69 via lead 71. The ringand sealing counter 69 is connected by lead 72 to the pulse widthmodulation logic network 45 and to the six-step wave forming and phaseangle control logic network 51. The appearance of an output on lead 72to pulse width modulation logic network 45 and six-step wave forming andphase angle control logic network 51 will allow either of these networksto be enabled whenever there is also an input signal on either lead 56or 57, respectively, from low pass filter 58 and high pass filter 59 ofthe mode selector 54. Reference is now made to FIG. 2 which illustratesthe voltage versus the frequency characteristic requirement of a typicalinduction motor. It will be noted that the initial inclinedportion ofthe upper starting and running curve is representative of constanttorque operation, that is, from zero to approximately 60 hertz thevoltage is varied as well as the frequency of the supply voltage. Theremaining portion 81 of the upper curve is representative of constanthorsepower wherein the voltage is held substantially constant while thefrequency of the motor supply voltage is steadily increased as thevehicle is accelerated. It has been found that voltage in the constanttorque range of operation may be effectively controlled by the pulsewidth modulation technique afforded by logic network 45, while voltagereduction in the constant horsepower range may be effectively controlledby phase shifting the two inverter outputs away from the nominal il5point. Smooth and efficient operation follows so long as supply power lsconstantly connected to terminal 43. In actual practice the terminal 43is connected to a pantograph (not shown) which picks off power from anoverhead catemary (not shown) or is connected to a conductor shoe (notshown) which rides on a third rail (not shown) in an electrified supplysystem. In previous motor control systems, excessive current or voltageon the catenary or on the third rail causes a tripout device tointerrupt the power to the motor by removing supply voltage fromterminal 43. Power may also inadvertently be removed by the motorman ifhe actuates the stop button which opens the power supply to theinverters, and thus removes excitation from the motor. Hence, each ofthese faults causes deenergization of the system and a concomitantchange in motor frequency, which frequency may be lower or higher thanthe normal power supply frequency dependent upon the load demands of themotor.

As mentioned above, let us assume that the motor 1 l is employed forpropelling a vehicle along a guide roadway, such as, a railway track. Itwill be seen that upon deenergization the rotational speed of the motormay increase dependent upon the grade of the trackway. If one attemptsto restart the induction motor 11 at the voltage and frequency valueswhich are present at the instant of loss of excitation, itis possible tocause system shutdown in that the protective devices, such as, the fusesor circuit breakers, will be blown by high current surges that mayappear. In the instant embodiment of the invention one such fuse isdesignated by reference numeral 50. The fuse 50 protects thesemiconductive devices of the inverters from being damaged or destroyed.The high current surges usually are produced by either one of twoconditions. For example, if the remanent magnetic state of the statorpoles is at or near the saturation level and if the instantaneous valueof voltage is near or at its peak value coupled with the fact that itsphase relationship is in the same relationship as that which existedprior to the loss of excitation, the effective impedance of the motorwinding coils will be very low or substantially zero so that high loadcurrent demands result. Similarly, if the frequency of the reappliedvoltage is different from the frequency of the rotating rotor, then thedifference frequency simulates slip and therefore the motor will demanda high torque effort which may also result in large current surges.Thus, in order to alleviate the high current surges, it is necessary toeffectively reduce the voltage level of the supply source and to changethe frequency of the inverter voltage to correspond to the frequencywhich is representative of rotor speed at the time power is applied atrestart. Such operation is represented by the lower curve 82 which islabeled as the restarting curve. It will be appreciated that by havingthe frequency tied in to motor synchronous speed the transient ofgetting the motor back into torque production is virtually eliminated.

NORMAL OPERATION Let us assume that the system is energized by closureof the switch 44 by the motorman. It will be noted that no rotationalmovement will be imparted to the rotor until the inverter'is turned onat minimum frequency voltage and the level of the motor current producestorque that exceeds the need of the system. It will be noted, as shownin FIG. 2, the voltage value and the frequency of motor voltage arelinearly increased in accordance with curve 80 corresponding to fullload on the motor. As previously mentioned, the lower response curveportion 80 is under control of the pulse width modulation control logicnetwork 45. The pulse width modulation control logic network 45 includesconventional integrated circuits which form necessary logic circuits,such as, gates, matrixes, amplifiers, and the like. The details of thepulse width modulation circuit are not necessary for an understanding ofthe present invention since any skilled programmer given the necessaryinputs could utilize Boolean algebra to formulate a software programwhich could be reduced to hardware circuits. Thus, any two givenprogrammers could solve the problem by two entirely different softwareprograms and hardware logic circuitry. It will be appreciated that thepulse width modulation network is enabled by signalsappearing on leads56-and 72. Thus, as the rotor begins turning and driving the tachometer64, signals proportional to the speed of the rotor will appear on lead67. Lead 61 applies the signals to the mode selector 54 and since theinitial frequency is relatively low, namely, 60 hertz or under, they arepassed by the low pass filter 58 so that an enabling control signal isdelivered to the pulse width modulation logic 45. The logic circuit 45provides a gating signal at select times to be delivered to the inputsof the inverter 41 over multiwire cables 46 and 48 and to the inverter42 over multiwire cables 47 and 49. In actual practice, the lowerportion of the starting curve is divided into a discrete number offrequency ranges. That is, it has been found advantageous to decreasethe notch rate of the pulse width modulations as the frequency isincreased at select points along the curve 80 in order to provide a moreeven voltage gradient. That is, the gating signals produced by the pulsewidth modulation network 45 will have a variable notch rate and pulsewidth in accordance with the frequency of operation so that theinverters will vary the effective voltage appearing on the motorwindings. A minor variation in effective voltage may be called for'bythe difference between heavy and light loading.

Turning now to FIG. 3, which is illustrative of pulse width modulation,it will be seen that the voltages on A, qSB and C are similarly shapedbut shifted 120 apart, and these voltage wave forms are of the typewhich will appear on leads LA, LB and LC. It will be further understoodthat two thyristors are employed for each phase and that each thyristorwould normally be turned on? and off in the timing sequence depicted inFIG. 4. However, with pulse width modulation each thyristor is renderednonconductive a number of times proportional to the notch rate, so thata series of output pulses are produced whose widths are proportional tothe remainder of the wave with notches removed. The number of notchesand the widths determine the number of voltage pulses that will appearon the output leads LA, LB and LC. In the example shown in FIG. 3, thesingle notch NA is approximately 30 wide so that an initial pulse of 75and a final pulse of 75 will appear every half cycle. That is, lead LAwill be at a 600 volt level for the initial 75 and will return to groundfor 30 duringthe width of notch NA, and will then go to the 600 voltlevel for the remaining 75 of the half cycle. On the alternate halfcycle, line LA will be grounded for the initial 75 and will go to the600 volt level for the next 30, which is the width of the notch NA, andwill then go to ground for the subsequent 75. The voltages on lines LBand LC undergo a similar transition except that they are 120"v out ofphase. Accordingly, the voltage appearing across lines LA and LB,namely, V (FIG. 3) is the sum of the voltage differences of phases qbAand B. Note that the fundamental of the voltage V is a quasi sinusoidalwave having a frequency equal to the frequency of the inverter andhaving an effective peak value which is proportional to the sum of thearea under the V voltage curve. Thus, by decreasing the notch width andincreasing the frequency of the inverter in proportion to the speed ofthe motor, the effective voltage to the first stator wye connectedwindings W1, W2, W3 is increased. In a similar fashion, the secondstator wye connected windings W4, W5, W6 fed by inverter 42 are likewisecontrolled. However, the fundamental sinusoidal wave 86 of the voltage Vis shifted 115 about the reference angle on the motor to compensate forthe mechanical displacement of the motor windings W4, W and W6, asearlier noted. The pulse width modulation operation will continue untilthe base speed of the motor is reached which has been selected to be atthe 60 hertz frequency. At that time the pulse width modulation logicnetwork 45 is disabled due to the lack of the signal on lead 56. Thatis, low pass filter 58 is unable to pass frequency signals higher than60 hertz. However, the high pass filter 59 now passes the higherfrequency signals produced by the tachometer 64 into lead 57. A signalon lead 57 enables the six-step wave forming network 51 so that the pairof three-phase inverters 41 and 42 are controlled by gating which nowappears on leads 52, 46 and 53, 47, respectively.

Thus in viewing FIG. 2, it will be noted that at approximately 60 hertzthe motor enters a constant horsepower mode of operation. It can be seenthat the voltage will remain constant while the frequency varies inaccordance with the speed of the rotor. In viewing FIG. 4, an analysisof the six-step operation will now be described in detail. As previouslymentioned, the inverter thyristors are turned on and off every 180 bythe logic circuit 51. Thus, the voltage on line LA is represented bycurve A, which shows the voltage at the full level of power supply forhalf a cycle and at ground level for the second half of the cycle.Similarly, curves B and qbC are rendered conductive and nonconductive onalternate half cycles. However, these latter voltages are respectivelyphase shifted 120. In a like manner, the outputs A', 4:8 and C' whichappear on leads LA, LB and LC of the inverter 42, have similar wavecharacteristics except they appear 30 later than outputs A, (B and C,respectively. It will be appreciated that the voltage across any givenpair of inverter output lines is the sum of the voltage on the line. Forexample, output voltage V which is representative of the voltage acrosslines LA and LB, is the sum of the voltages d) A and (#8, as shown inFIG. 4. Similarly, the voltage V, as shown in FIG. 4, is the sum of thevoltages qSA and B'. It will be noted that each of the voltages V and Vtakes the form of a six-step wave and has a frequency equal to theswitching rate of the inverters 41 and 42. Thus, by varying theswitching frequency of the inverters, the frequency of the fundamental88, 89 as well as the resultant line-to-neutral voltage represented bycurve 90 shown here in dotted fashion can be varied. The voltage waveVWl follows the equation The other motor winding voltage follows asimilar equation. 7

In order to more closely approximate the first harmonic fundamentalwave, it has been found to be advantageous to transpose the six-stepwave form into a twelve-step wave form. This is accomplished by thewindings of the reactor 12 which parallel connect the inverters to thetwo three-phase wye connected stator windings of motor 11. It will beseen that the resultant fundamental wave form 90 shown superimposed ontwelve-step wave form V is applied to winding W1. It should be notedthat each of the windings of the motor is fed with similar voltagesexcept that the phase angles remain in the same relationship asdescribed I above. It will be appreciated that, as the frequency isvaried over the constant horsepower range, the peak value of fundamentalvoltage remains unchanged.

Thus, at the lower end of the running motor characteristic curve, themotor is operated as a constant torque device and therefore after thebase speed point is reached the motor is operated as a constanthorsepower device.

Returning now to a review of FIG. 3, it will be assumed that the motoris operating in the constant torque range and that a loss of excitationoccurs due to a voltage surge on the line or the like. Let us assumethat the motor is operating at the hertz point on portion 80 of therunning curve when the loss takes place. As previously mentioned, twoconditions are required in order to prevent unduly high current surgesfrom causing deleterious effects on the system. First it is necessary toreduce the voltage level as well as to readjust the frequency of thevoltage in order to prevent high surge currents from being generatedupon restart. Under the assumed condition the system is under control ofthe pulse width modulation logic network so that at the time of thepower loss, the network is programmed to the operation of the restartingcurve as shown in FIG. 2. The interrupted power causes the pulse widthmodulation logic network 45 to control the level of voltage from point91 to substantially point 92 so that the amplitude of the invertervoltages is dramatically reduced to a level which will ensure that highcurrent curges will not be generated upon restart. This phenomena isaccomplished by increasing the notch taken out of the wave form. If thenotch wave form is of the type shown in FIG. 3, the width of the notchNA is increased a preselected amount. Obviously the increase in notchedwidth results in an inherent reduction in the peak value of thefundamental voltage applied to the windings of motor 11. Two suchfundamental voltage wave forms 85 and 86 are shown superimposed onvoltages V and V, respectively. The resultant fundamental wave form 87is also reduced. It will be appreciated that the width of the notch ornotches is dependent upon the range in which the motor is operating atthe time of the occurrence of the loss of excitation. If there arenumerous notches, each notch width need only be increased slightly todramatically reduce the average value of voltage.

As mentioned, the notch rate varies over a range within the portion ofthe constant torque curve 80 so that in effect the notch rate of anyparticular range does not vary but simply the widths .of the notchesvary to allow for the effective peak value of the fundamental to bereduced to a level which will ensure that no surge currents are producedduring peak value. Further, as mentioned above, it is mandatory that thefrequency of the rotating rotor be measured at the time of reapplicationof power so that no torque demand be required upon restart in order toprevent the generation of current surges. Thus, the tachometer 64measures the speed of the rotor R and produces a signal indicative ofthe frequency of rotation which appears on leads 67, 61, 62; is passedby low pass filter 58 and lead 56 and effectively controls the pulsewidth modulation logic network 45, and in turn causes the frequency ofswitching of the inverters 41 and 42 to correspond with the frequencywhich is representative of the speed of the rotor R. This effectivelyensures that no demand for torque will be initially required for restartoperation in the constant torque range. After the initial reduction ofthe voltage applied to the windings of the motor 11, the voltage israpidly raised to its normal value by quickly decreasing the width ofnotch NA as shown in FIG. 3. The motor assumes normal operation andfollows the upper curve again if full torque is required and no furtherinterruption is encountered.

Referring now to FIG. 5, which simulates the motor operating in theconstant horsepower range, it will be appreciated that a failureoccurring during this operation also requires that the voltage level bereduced and that the frequency of the rotating rotor be measured at thetime of restart. Thus, if a failure occurs during constant horsepoweroperation, it is required that the system assume restart operation at apoint along the restarting curve 82. That is, the voltage level must bereduced from full power to a substantially lower value as is representedby curve 82. This operation is accomplished by means of phase shiftingthe respective phase angles of the voltages of the inverter inrelationship to each other. It has been found that a phase shift ofapproximately 150 will substantially reduce the winding voltage on themotor to a level which will prevent surge currents from being generated.For example, as shown in FIG. 5, a phase shift of only 60 dramaticallyreduces the peak voltage of the fundamental curve 93 that is applied towinding W1. Thus, upon occurrence of a loss of excitation, the sixthstep wave form and phase angle control logic network 51 produce a signalwhich phase shifts the inverter 41 in one direction and phase shifts theinverter 42 in the opposite direction. As seen in FIG. 5, the phaseshift about the motor reference accomplishes the necessary reduction inthe amplitude of the voltage which is applied to the windings of themotor 11. As in the case of the constant torque opera tion thetachometer 64 measures the actual rotational speed of the rotor R andthe tachometer 64 applies a frequency signal which is representative ofthe rotational speed of the motor. Thus, the step wave form and phaseangle control logic network 52, in turn, controls the switchingfrequency of the inverters 41 and 42. Thus, no torque demand due todeviation of motor speed from synchronous speed of the motorll will takeplace upon restart. It will be understood that the reduction of thevoltage is only momentary and upon reapplication of the power thesix-step wave form and phase angle control logic network 51 willimmediately take effect and rapidly increase the voltage to full valueso that normal operation may be resumed.

Thus, when the presently described control system is employed in arailroad mass and/or rapid transit operation, little, if any, delay isincurred upon the reapplication of power after the occurrence of a lossof excitation. Thus, maximum-service and operation are realized and amost efficient type of motor control function is produced.

While the invention has been described in relation to a transportationsystem, it will be appreciated that the control system may be utilizedin other environments which utilize induction motors. Further, it isunderstood that while only a single motor has been illustrated, thecontrol system is, in fact, capable of supplying power to a multitude ofinduction motors. Generally, a moving vehicle includes a drive motor oneach axle of the vehicle and therefore a single control system tooperate the motors in parallel.

It will be understood that other changes may be made to the presentlydescribed control system without departing from the spirit and scope ofthe invention and,.

therefore, these modifications, variations and alterations are meant tobe covered by the subject matter of the annexed claims.

Having thus described our invention, what we claim l. A control systemfor reenergizing a rotating induction motor comprising, a system ofconverters for supplying polyphase voltage to said induction motor, aspeed sensing means for measuring the speed of said induction motor, anda .programmed logic circuit means coupled to said converter system andresponsive to said speed sensing means for causing the frequency of saidpolyphase voltage to substantially agree with the frequency which isrepresentative of the synchronous speed of said induction motor at thetime that said polyphase voltage is reapplied to said induction motorand reducing the amplitude of said polyphase voltage which is reappliedto said induction motor.

2. The control system as defined in claim I, wherein said system ofconverters includes a pair of inverters one of which is phase shifted inone direction with respect to a reference point and the other of whichis phase shifted in the other direction with respect to the referencepoint.

3. A control system for dynamically reenergizing a rotating inductionmotor comprising, a pair of threephase inverters for supplying polyphasevoltage to said induction motor through a reactor, a .speed sensingmeans for measuring the speed of said induction motor, and a programmedlogic circuit coupled to said pair of inverters and responsive to saidspeed sensing means for decreasing the amplitude of said polyphasevoltage when an interruption occurs in the system and for readjustingthe frequency of said polyphase voltage to the frequency which isrepresentative of the synchronous speed of said induction motor at thetime that said polyphase voltage is reapplied to said induction motor.

4. The control system as defined in claim 3, wherein said inductionmotor includes a pair of three-phase wye connected windings and asquirrel-cage rotor.

5. The control system as defined in claim 3, wherein said pairofthree-phase inverters are phase shifted relative to each other in orderto eliminate the fifth and seventh harmonics of the fundamental of saidpolyphase voltage.

6. The control system as defined in claim 3, wherein said reactorparallels said induction motor and said pair of three-phase inverters. v

7. The control system as defined in claim 3, wherein said programmedlogic circuit includes a pulse width modulation network for varying theamplitude of said polyphase voltage produced by said pair of three-phaseinverters.

8. The control system as defined in claim 3, wherein said logic circuitincludes a phase angle control network for varying the amplitude of saidpolyphase voltage produced by said pair of three-phase inverters.

9. The control system as defined in claim 3, wherein said inductionmotor includes a pair of three-phase wye connected windings which aremechanically displaced with respect to each other.

10. The control system as defined in claim 9, wherein said pair ofthree-phase inverters have outputs which are electrically shiftedrelative to each other.

11. The control system as defined in claim 10, wherein said mechanicaldisplacement of said pair of wye connected windings is substantiallyequal to the electrical shift of said outputs of said pair of threephaseinverters.

12. The control system as defined in claim 3, wherein said inductionmotor includes a pair of three-phase windings which are parallel coupledto said pair of three-phase inverters by said reactor.

13. The control system as defined in claim 12, wherein said reactorincludes a plurality of main windings and a plurality of auxiliarywindings.

14. The control system as defined in claim 13, wherein said main windingof one phase is associated with an auxiliary winding of another phase.

15. The control system as defined in claim 3, wherein said programmedlogic circuit includes a six-step wave form control network for .varyingthe frequency of said polyphase voltage.

16. The control system as defined in claim 8, wherein said phase anglecontrol network shifts the phase angle of the output of one of siad pairof three-phase inverters forward a given amount and shifts the phaseangle of the output of the other one of said pair of threephaseinverters backward the same amount.

17. The control system as defined in claim 3, wherein said inductionmotor includes a first and a second wye connected winding arrangementwhich are parallel coupled to respective ones of said pair of polyphaseinverters by said reactor which includes a multitude of windings.

18. A system fo starting an d running an induction motor and forreenergizing an indiicfiofimdtor in it? tion comprising, a dc. voltagesource, a pair of phase shifted inverters supplied by said dc. voltagesource, an inductive reactor coupling an output of said pair of phaseshifted inverters to the stator windings of said induction motor, atachometer driven by the rotor of said induction motor, a programmedlogic circuit responsive to said tachometer for controlling the outputof said pair of inverters so that said induction motor is operated as aconstant torque device over a portion of its operating range and so thatthe induction motor is operated as a constant horespower device over theremaining portion of its operating range.

19. The system as defined in claim 18, wherein said induction motorincludes a squirrel cage rotor.

20. The system as defined in claim 19, wherein said pair of invertersare connected in parallel with the dual three-phase wye connectedwindings by said inductive reactor.

21. The system as defined in claim 18, wherein said inductive reactorincludes a plurality of main windings and a plurality of auxiliarywindings one of which is intercoupled with one of said main windings.

22. A system comprising,

a. a polyphase induction motor,

b. a reactor,

0. a polyphase inverter,

d. a do supply source,

e. a logic circuit,

f. a speed sensing means,

said polyphase induction motor including a pair of mechanicallydisplaced three-phase wye connected stator windings of main windings anda plurality of auxiliary windings connected to said stator windings,said polyphase inverter including a pair of electrically displacedthree-phase switching sections connected to said main windings, saidd.c. supply source providing power for said polyphase inverter, saidlogic circuit including a pulse width modulation section and a phaseangle control section which are connected to said polyphase inverter,said speed sensing means measuring the speed of said squirrel cage rotorand causing said logic circuit to pulse width modulate the output ofsaid inverter so that said polyphase induction motor initially runs as aconstant torque device on startup and upon reaching a base speed saidlogic circuit causes the output of said inverter to produce a steppedwave voltage which thereafter allows said polyphase induction motor torun as a constant horsepower device, said speed sensing meanscontrolling said logic circuit in a manner such that the amplitude ofthe output of said polyphase inverter is decreased and that thefrequency of the output of said polyphase inverter is made tosubstantially correspond with the frequency of the synchronous speed ofthe rotating rotor of said polyphase induction motor.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION atent No.3,781,616 Dated December 25, 1973 humor) Boris Mokrytzki and Peter W.Hammond It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

r- Column 13, line 22, "siad" should be --said-- w Col umn 11;, line 19,after "windings", first occurrence,

insert --a.nd a squirrel cage rotor,

said reactor including a plurality-- Signed and sealed this 23rd day ofJuly 1971 (SEAL) v Attest: I

McCOY M. GIBSON,Q JR. 0. MARSHALL DANN Attesting Officer Commissioner ofPatents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3 ,781,616 mu December 25, 1973 I v Boris Mokrytzki and Peter W. HammondIt is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

'- Column 13, line 22, "siad" should be --ss.id--

Coiumn 11 line 19, after "windings", first occurrence;

insert and a squirrel cage rotor,

said reactor including a plurality-- (SEAL) Attest:

MCCOY M. GIBSON, JR. 0. MARSHALL DANN Attesting Officer 7 Commissionerof Patents

1. A control system for reenergizing a rotating induction motorcomprising, a system of converters for supplying polyphase voltage tosaid induction motor, a speed sensing means for measuring the speed ofsaid induction motor, and a programmed logic circuit means coupled tosaid converter system and responsive to said speed sensing means forcausing the frequency of said polyphase voltage to substantially agreewith the frequency which is representative of the synchronous speed ofsaid induction motor at the time that said polyphase voltage isreapplied to said induction motor and reducing the amplitude of saidpolyphase voltage which is reapplied to said induction motor.
 2. Thecontrol system as defined in claim 1, wherein said system of convertersincludes a pair of inverters one of which is phase shifted in onedirection with respect to a reference point and the other of which isphase shifted in the other direction with respect to the referencepoint.
 3. A control system for dynamically reenergizing a rotatinginduction motor comprising, a pair of three-phase inverters forsupplying polyphase voltage to said induction motor through a reactor, aspeed sensing means for measuring the speed of said induction motor, anda programmed logic circuit coupled to said pair of inverters andresponsive to said speed sensing means for decreasing the amplitude ofsaid polyphase voltage when an interruption occurs in the system and forreadjusting the frequency of said polyphase voltage to the frequencywhich is representative of the synchronous speed of said induction motorat the time that said polyphase voltage is reapplied to said inductionmotor.
 4. The control system as defined in claim 3, wherein saidinduction motor includes a pair of three-phase wye connected windingsand a squirrel-cage rotor.
 5. The control system as defined in claim 3,wherein said pair of three-phase inverters are phase shifted relative toeach other in order to eliminate the fifth and seventh harmonics of thefundamental of said polyphase voltage.
 6. The control system as definedin claim 3, wherein said reactor parallels said induction motor and saidpair of three-phase inverters.
 7. The control system as defined in claim3, wherein said programmed logic circuit includes a pulse widthmodulation network for varying the amplitude of said polyphase voltageproduced by said pair of three-phase inverters.
 8. The control system asdefined in claim 3, wherein said logic circuit includes a phase anglecontrol network for varying the amplitude of said polyphase voltageproduced by said pair of three-phase inverters.
 9. The control system asdefined in claim 3, wherein said induction motor includes a pair ofthree-phase wye connected windings which are mechanically displaced withrespect to each other.
 10. The control system as defined in claim 9,wherein said pair of three-phase inverters have outpuTs which areelectrically shifted relative to each other.
 11. The control system asdefined in claim 10, wherein said mechanical displacement of said pairof wye connected windings is substantially equal to the electrical shiftof said outputs of said pair of three-phase inverters.
 12. The controlsystem as defined in claim 3, wherein said induction motor includes apair of three-phase windings which are parallel coupled to said pair ofthree-phase inverters by said reactor.
 13. The control system as definedin claim 12, wherein said reactor includes a plurality of main windingsand a plurality of auxiliary windings.
 14. The control system as definedin claim 13, wherein said main winding of one phase is associated withan auxiliary winding of another phase.
 15. The control system as definedin claim 3, wherein said programmed logic circuit includes a six-stepwave form control network for varying the frequency of said polyphasevoltage.
 16. The control system as defined in claim 8, wherein saidphase angle control network shifts the phase angle of the output of oneof siad pair of three-phase inverters forward a given amount and shiftsthe phase angle of the output of the other one of said pair ofthree-phase inverters backward the same amount.
 17. The control systemas defined in claim 3, wherein said induction motor includes a first anda second wye connected winding arrangement which are parallel coupled torespective ones of said pair of polyphase inverters by said reactorwhich includes a multitude of windings.
 18. A system for starting andrunning an induction motor and for reenergizing an induction motor inmotion comprising, a d.c. voltage source, a pair of phase shiftedinverters supplied by said d.c. voltage source, an inductive reactorcoupling an output of said pair of phase shifted inverters to the statorwindings of said induction motor, a tachometer driven by the rotor ofsaid induction motor, a programmed logic circuit responsive to saidtachometer for controlling the output of said pair of inverters so thatsaid induction motor is operated as a constant torque device over aportion of its operating range and so that the induction motor isoperated as a constant horespower device over the remaining portion ofits operating range.
 19. The system as defined in claim 18, wherein saidinduction motor includes a squirrel cage rotor.
 20. The system asdefined in claim 19, wherein said pair of inverters are connected inparallel with the dual three-phase wye connected windings by saidinductive reactor.
 21. The system as defined in claim 18, wherein saidinductive reactor includes a plurality of main windings and a pluralityof auxiliary windings one of which is intercoupled with one of said mainwindings.
 22. A system comprising, a. a polyphase induction motor, b. areactor, c. a polyphase inverter, d. a d.c. supply source, e. a logiccircuit, f. a speed sensing means, said polyphase induction motorincluding a pair of mechanically displaced three-phase wye connectedstator windings and a squirrel cage rotor, said reactor including aplurality of main windings and a plurality of auxiliary windingsconnected to said stator windings, said polyphase inverter including apair of electrically displaced three-phase switching sections connectedto said main windings, said d.c. supply source providing power for saidpolyphase inverter, said logic circuit including a pulse widthmodulation section and a phase angle control section which are connectedto said polyphase inverter, said speed sensing means measuring the speedof said squirrel cage rotor and causing said logic circuit to pulsewidth modulate the output of said inverter so that said polyphaseinduction motor initially runs as a constant torque device on startupand upon reaching a base speed said logic circuit causes the output ofsaid inverter to produce a stepped wave voltage which thereafter allowssaid polyphase induction motor to run as a constAnt horsepower device,said speed sensing means controlling said logic circuit in a manner suchthat the amplitude of the output of said polyphase inverter is decreasedand that the frequency of the output of said polyphase inverter is madeto substantially correspond with the frequency of the synchronous speedof the rotating rotor of said polyphase induction motor.